Powdered metal is used to fabricate tools and components because it can provide a finished product that does not require extensive machining to achieve the desired surface finish. A mandrel, fabricated to a desired geometrical shape, is used for fabricating hot isostatic pressed (HIP) powder metal tooling. The mandrel must be geometrically accurate and produce an excellent surface finish to minimize the amount of further machining required to finish the tool.
A conventional electrodeposition process for producing a metal electrodeposition product duplicates the surface configuration of an object. In the process a model is made of an original or desired object and metal is electrodeposited onto the model's contacting surface (i.e., molds working surface) which is a negative of the original object. The model is removed leaving the deposited metal with the front metal surface, the surface that was in contact with the mold, replicating the surface of the original object. When the metal is deposited in a thin layer, a backing material is added to the back metal surface, the surface that was not in contact with the mold, to provide stability and strength to the metal. The backing material is added before separating the deposited metal from the mold. This process has been used for replicating works of art, and while it duplicates fine surface details, it is painfully slow requiring time to apply and remove the backing material in addition to the several days required to deposit metal to a thickness of one sixteenth of an inch.
Another process, for manufacturing a three-dimensional electroformed mold shell, uses a model onto which molten epoxy resin is poured to form an inverted model shaped like a shell. Double sided adhesive tape is applied to the front surface of the shell that was in contact with the model. A conductive network body is applied on the tape and deformed into a three-dimensional shape so as to adapt itself to the shape of the front shell surface. The network body forms the cathode for electrodeposition. The process is interrupted to remove the network body from the tape and inverted shell thereby adding more time to the several days required to complete electrodeposition.
Laser sintering may also be employed to fabricate a metal article. The article is produced in layerwise fashion from a laser fusible powder that is dispensed one layer at a time. A difficulty with this process is that the article formed is not fully dense and retains some porosity which limits its structural strength. After fabrication of the article, the laser is again used to apply a gas impervious skin to the article for use in a hot isostatic pressing (HIP) process to densify the article. The dimensional resolution and shape complexity of articles produced by HIP processes are somewhat limited because shrinking occurs during the process. This technology requires special laser sintering equipment.
Quite understandably, production of the mandrel is one of the most important steps in the process of tool making because a poor mandrel will produce a poor tool requiring expensive machining to obtain the desired surface finish. On the other hand, a good mandrel yields the greatest possible savings in cycle time in tool production. Machined mandrels have excellent surface finishes but machining is slow and increases cycle time in tool production. Ceramic molds can be used but they do not provide the very fine surface finishes required for tool making. Accordingly, it will be appreciated that it would be highly desirable to have a method for producing a mandrel for use in tool making that provides the very fine surface finish required without further machining and that lends itself to rapid tool making.